In the preparation of such materials as polyurethane, certain paints, varnishes, epoxies and the like from a plurality of liquid components, it is generally necessary to maintain a selected volume ratio between the components. The ratio can vary depending upon the particular application and on the chemical formulation of a given component. For example, in the case of polyurethane, the two components, isocyanate and polyol, are prepared in a variety of formulations depending upon the application. Such formulations are often intended to be combined in a 1:1 ratio. However, other mixing ratios, such as 5:1 and 1:5, are not uncommon. The appropriate mixing ratio for any particular application may also vary with environmental conditions such as temperature, which affect the reactivity, viscosity or other physical or chemical properties of the components of the mixture.
During the delivery of fluid components, the actual ratio between the components as delivered may vary from a desired, preset value, for example, as a result of a blockage in a hose. Therefore, the delivery of components needs to be monitored and occasionally corrected.
It is desirable for an operator of a plural component delivery system to be able to adjust the rate of flow of mixed material from a mixing device during operation. During and after such adjustments, the ratio between the components as they are being delivered from their respective sources to the mixing device should remain constant. It is also desirable for an operator to be able easily to adjust in very small increments the proportional output of respective pumps that deliver the liquid components, to compensate for any day to day changes that may occur in the properties of the components. It is also desirable that a plural component delivery system allow the selection of a component ratio from a wide range of values.
Accordingly, there is a need for a mixing and delivery system in which the ratios between the plurality of liquid components can be accurately set, maintained and changed.
In plural liquid component applications such as the mixing of paint colors, it may be desirable to keep the individual paint components, which do not react with each other chemically, separate from each other.
In contrast, in the creation of certain materials from plural liquid components, the components react with each other upon mixing, although they are individually relatively stable. The speed of such reaction, and consequently the formation of the resulting product, depends upon the particular chemical formulations of the components and on ambient conditions. In some cases, it can be quite rapid. An example is the spraying of a polyurethane mixture on a surface that does not allow for containment of the mixture during gelling or curing, for example, on the bed of a pick-up truck. Here, as well as in certain other types of applications, it is not convenient to apply heat to accelerate the reaction of the components. Thus, the isocyanate and polyol preparations intended for use in the spraying of polyurethane are formulated to react aggressively when mixed , together, to form the polyurethane quickly. It is necessary to keep the components isolated from each other prior to mixing. Precise and proportionate volumes of the components must be delivered to the mixing device simultaneously, maintaining a preselected ratio. For the proper reaction to take place, complete mixing of the components must occur prior to spraying. Once properly mixed, the reacting fluid material must be delivered from the mixing device at a rate fast enough that it does not gel prematurely and obstruct the flow from the device.
There are numerous types of systems designed for the mixing and delivery of plural liquid components. Some of the most common machines for the spraying of polyurethane use piston type pumps with a separate cylinder for each component of the mixture. These systems must be operated at high pressure (above approximately 1000 psi) to minimize surging line pressure during the piston strokes and to permit consistent impingement mixing in the spray gun. Changing the delivery ratio in such systems generally requires substitution of a different sized pump, which obviously cannot be done while the system is operating, and which involves significant "down-time" of the machine. Smaller adjustments may in some cases be .made using flow valves. In general, these systems are relatively expensive.
Less costly are low pressure (below approximately 1000 psi) application systems that typically use positive displacement rotary vane or rotary gear pumps. Here, one motor is often used to drive two or more gear pumps, with the relative rates of component flow being set by the gear ratio between pumps. A pre-set ratio of component volumes is thereby maintained despite any fluctuations that may occur in motor speed. If a small change in the ratio is desired, it can sometimes be accomplished using control valves in the lines between the pumps and the mixing/delivery device. However, this is generally not desirable, as it requires recalibration of the system. It is more common to change the gear ratio of the pumps, which often involves substituting pump drive gears, or even one pump for another. Consequently, low pressure systems are not commonly used when variable rate control is desired.
U.S. Pat. No. 4,809,909 describes a high pressure system for applying plural reactive liquid components. A single air-driven piston motor simultaneously drives respective piston pumps. The piston pumps may be individually adjusted to produce a desired rate of component flow.
U.S. Pat. No. 3,232,585 describes a two-component foam spray system using two motors that are directly coupled through a gear box. The relative speeds of the motors are set by choosing an appropriate gear ratio. The ratio between the motor speeds cannot be altered while the system is running. U.S. Pat. No. 4,789,100 describes a system for pumping at least two fluids in a desired ratio, in which a single common drive motor is mechanically coupled through a gear reduction system to each fluid's respective pump.
U.S. Pat. No. 4,019,653 describes a paint spray system in which the relative proportions of the components are monitored using respective flow sensors, and set using computer control of respective valves. Such flow sensors are relatively expensive. Maintenance of these components, which are downstream of the pumps, requires disassembly of the system.
U.S. Pat. No. 3,921,901 describes a system for the atomization of a liquid spray having two components. The components are delivered to an emulsifier by respective pumps, their ratio being set using respective flow sensors. The means driving the pumps is not specified.
U.S. Pat. No. 4,998,672 describes a liquid sprayer in which flow rate can be manually controlled according to the position of a trigger on the sprayer. When used with a two-component mixture, the flow of each component is driven by a respective motor, and flow rate is monitored by a respective sensor. A computer processes signals from the flow sensors and sends feedback control signals accordingly to the respective motors to maintain the flow rates in a predetermined ratio.